Add definition of separator used for logger names

[dhood]

rcutils logging has logger names as "a.b.c". this breaks out the dot separator so it doesn't have to be hard-coded here and elsewhere. For example, in rcl, we are using the separator when changing /namespace/node to logger name namespace.node.

Since some callers might want a char literal (e.g here) and some might want a string literal so it's null terminated (e.g. here), I've provided definitions for both. This seems like a bit of an unusual thing to do though, so I'm open to alternatives 😄

[wjwwood]

makes sense to me, +1

[dhood]

CI on this was included in ros2/rcl#212 (comment)